The present invention generally relates to the repair of components made from material such as metals, composites, wood, plastics, glass and other materials. It is to be appreciated that the present invention has general and specific industrial application in the repair of various materials. The term “composite” is commonly used in industry to identify components produced by impregnating a fibrous material with a thermoplastic or thermosetting resin to form laminates or layers.
Generally, polymers and polymer composites have the advantages of weight saving, high specific mechanical properties, and good corrosion resistance which make them indispensable materials in all areas of manufacturing. Nevertheless, manufacturing costs are sometime detrimental, since they can represent a considerable part of the total costs and are made even more costly by the inability to quickly and easily repair these material without requiring a complete, and expensive, total replacement. Furthermore, the production of complex shaped parts is still a challenge for the composite industry. The limited potential for complex shape forming offered by advanced composite materials leaves little scope for design freedom in order to improve mechanical performance and/or integrate supplementary functions. This has been one of the primary limitations for a wider use of advanced composites in cost-sensitive large volume applications. Additionally, the nature of composite materials does not lend itself to easy repair, especially on cheap, mass produced items and repair kits for more expensive, specialty items (such as in the aeronautic industry) are bulky, expensive, and require long time to complete the repair.
Shape memory polymers (SMPs) and shape memory alloys (SMAs) were first developed about 20 years ago and have been the subject of commercial development in the last 10 years. SMPs are polymers that derive their name from their inherent ability to return to their original “memorized” shape after undergoing a shape deformation. SMPs that have been preformed can be deformed to any desired shape below or above its glass transition temperature (Tg). If it is below the Tg, this process is called cold deformation. When deformation of the SMP occurs above its Tg, the process is denoted as warm deformation. In either case the SMP must remain below, or be quenched to below, the Tg while maintained in the desired deformed shape to “lock” in the deformation. Once the deformation is locked in, the polymer network cannot return to a relaxed state due to thermal barriers. The SMP will hold its deformed shape indefinitely until it is heated above its Tg, whereat the SMP stored mechanical strain is released and the SMP returns to its performed state.
SMPs are not simply elastomers, nor simply plastics. They exhibit characteristics of both materials, depending on their temperature. While rigid, an SMP demonstrates the strength-to-weight ratio of a rigid polymer; however, normal rigid polymers under thermal stimulus simply flow or melt into a random new shape, and they have no “memorized” shape to which they can return. While heated and pliable, an SMP has the flexibility of a high-quality, dynamic elastomer, tolerating up to 400% elongation or more; however, unlike normal elastomers, an SMP can be reshaped or returned quickly to its memorized shape and subsequently cooled into a rigid plastic.
Several known polymer types exhibit shape memory properties. Probably the best known and best researched polymer types exhibiting shape memory polymer properties are polyurethane polymers. Gordon, Proc of First Intl. Conf. Shape Memory and Superelastic Tech., 115-120 (1994) and Tobushi et al., Proc of First Intl. Conf. Shape Memory and Superelastic Tech., 109-114 (1994) exemplify studies directed to properties and application of shape memory polyurethanes. Another polymeric system based on crosslinking polyethylene homopolymer was reported by S. Ota, Radiat. Phys. Chem. 18, 81 (1981). A styrene-butadiene thermoplastic copolymer system was also described by Japan Kokai, JP 63-179955 to exhibit shape memory properties. Polyisoprene was also claimed to exhibit shape memory properties in Japan Kokai JP 62-192440. Another known polymeric system, disclosed by Kagami et al., Macromol. Rapid Communication, 17, 539-543 (1996), is the class of copolymers of stearyl acrylate and acrylic acid or methyl acrylate. Other SMP polymers known in the art include articles formed of norbornene or dimethaneoctahydronapthalene homopolymers or copolymers, set forth in U.S. Pat. No. 4,831,094. Additionally, styrene copolymer based SMPs are disclosed in U.S. Pat. No. 6,759,481 which is incorporated herein by reference.
Modern aircraft are perhaps one of the largest users of composite materials. Composites are widely used in the aerospace industry to provide aircraft components such as fuselages, wings and tail fins, doors and so on. This is because composite components have the physical attribute of being relatively lightweight while at the same time having high structural strength in comparison to metals. Such composite components typically are of a sandwich construction. When damage occurs to such structures, for example by impacted damage from a flying stone or other debris or from a dropped tool, a damage crater, crack, or hole will be formed in the object concerned.
The general approach to repair damage is to remove the damaged part from the aircraft, and repair the damage by using an electric blanket with a vacuum bag. A “prepreg” formed of a layer of fibrous material impregnated with uncured resin is laid over the area to be repaired. The electric blanket applies heat to that area to cure the prepreg. The vacuum bag holds the electric blanket in position over the repair area while at the same time applying a compaction force to the prepreg.
Repairs using this approach are not however always satisfactory. This is because the inconsistency of the heat provided by the electric blanket leads to unreliability in the curing. Also, the use of vacuum bag compaction is not very effective in removing air from the prepreg so that the repaired area is not necessarily void free. Additionally, it normally takes a long amount of time to remove, repair, replace, and test the damaged component on an aircraft. Finally, the majority of time in using these methods typically involves waiting for the resin in the composite material and filler to cure. If this cure cycle was eliminated not only would there be a vast reduction in time but also in the emissions and use of chemicals, eliminating the cleanup and disposal of said chemicals.
A similar method of repair to such composite structures generally entails a lightweight composite filler material being inserted into the crater in a thixotropic state to protrude slightly from the outer surface. The filler is then allowed to harden and cure. It is then abraided flush with the surface of the structure. A patch of fiber reinforced composite material in either cured or more generally uncured state is then adhered to the surface of the structure over the filled crater using a separate adhesive and the patch is then bonded in position using both vacuum and heat. The vacuum is normally applied using an airtight sheet of material placed over the repair and temporarily sealed to the structure using a bead of adhesive around its periphery. A vacuum is then created under the sheet to try to ensure that any air bubbles are expelled from underneath the patch and to ensure good bonding. At the same time a heater blanket positioned inside or outside the vacuum bag will apply heat to the repair to effect hardening and curing of the adhesive which is normally a curable resin.
Multi-layered repair patches are also known in the art and these repair patches have been used both for repairing holes in drywall material as well as repairing holes in automobile bodies. U.S. Pat. Nos. 5,075,149 issued to Owens et al. (“Owens”), 4,707,391 issued to Hoffmann (“Hoffmann '391”) and 4,135,017 issued to Hoffmann (“Hoffmann '017”) are all directed to multi-layer repair patches.
Owens discloses a three-layered patch with a metal plate disclosed between two polyester sheets. The metal plate is held in place between the two polyester sheets with a semi-solid adhesive such as urethane. The semi-solid adhesive fixedly attaches the two polyester sheets together as well as fixedly attaching the reinforcing metal plate between the two sheets. Owens is not useful for repairs which require the application of bonding material or plaster to the repair patch because the bonding material or plaster cannot readily pass through the mesh due to the presence of the urethane adhesive. Additionally the patch cannot be molded quickly, on-site, without additional time and equipment.
Hoffmann '391 discloses a two-layer patch including a perforated metal plate with an outer fiberglass mesh attached to one side of the plate. A glue or adhesive coating is applied to the surface of the plate that is attached to the surface to be repaired and an additional adhesive coating is applied to the inward-facing surface of the fiberglass mesh to attach the mesh to the metal plate as well as to attach the mesh to the surface under repair.
Hoffmann '017 also discloses a two-layer patch. An inner metal plate is covered with adhesive that secures one surface of the plate to the surface under repair. An outer plate cover is laminated onto the exterior side of the metal plate by means of a layer of adhesive applied to the inward-facing side of the plate cover.
Both of these methods employ metal plates in the final patch with limits the ability of these patches to be easily and quickly molded to the damaged part on-site. Additionally, the use of metal eliminates some of the weight saving advantages of a pure composite repair patch.
Additionally, the repairs alone in these methods can take anywhere from approximately four hours or more to complete, mainly due to the time necessary to allow curing of the filler and adhesive. When taking into account the time to remove the damaged parts, mold the patch to the damaged area, and replace the part the time involved increases. In addition, despite the use of vacuum equipment to attempt to expel all air entrapped under the patch, the complete absence of such entrapment cannot be guaranteed and non-destructive testing may need to be carried out to ensure the structural integrity of the repair. With aircraft downtime often running at $US100,000.00 per hour it will be appreciated that enormous potential savings are possible when employing the method of the instant invention.
Additionally, if mass produced items, such as car hoods, bumpers, and other manufactured parts are damaged, it is oftentimes less expensive to replace the entire part than to repair it, although such parts are often expensive themselves. Thus there is a need for a cheap, quick, and effective method of repairing such mass produced parts and for quickly and reliably repairing aircraft and other high end parts.
It is the object of the present invention to provide a preformed and cured patch and a method to quickly and cheaply permanently repair any number of items with composite materials which retain similar or greater mechanical properties of the parts repaired. Another object is to provide a method for quickly and cheaply joining two parts together in order to form a larger part which retains similar mechanical properties of the original parts. These and other objects of the present invention will become apparent from the following specification.